The field of the present invention relates to systems for indicating failure of equipment, and more particularly to such systems for giving advance warning of equipment failure.
The Reliability and Maintainability Indicator (RAMI) was invented to overcome problems currently existing as described in the following paragraphs:
One of the most challenging procedures in a process plant is to establish the proper time interval for scheduling preventive maintenance. Various philosophies have prevailed such as using experience, past practices, mechanic's judgement, manufacturer's recommendations, or not doing preventive maintenance at all. This latter school of thought emanates from the conviction that it is better to operate equipment until it fails than to accept the maintenance and penalty costs of shutting down prematurely. Other approaches have weighed the cost of shutdown versus the risk of continuing in operation. This has been done by using state of the art statistical methods with data processed by computer programs which print out the scheduled maintenance time. All of these approaches are based either on a statistical or practical approach but in no sense provide an optimum time of scheduling preventive maintenance as is possible with RAMI. To illustrate:
The most recent sophisticated trend has been to use failure and Maintenance data generated from a Failure and Maintenance Reporting System to calculate the probability of an equipment failing due to a degradation or wear out type of failure. This methodology has received wide acclaim and has been one of the most meritorious of all statistical procedures used. Basically it depends on an analysis of the Normal Statistical Distribution, or in the general case, on the Weibull Distribution. By means of these distributions it is possible to predict the probability of an equipment failing prior to, on or before, a certain time. This time is usually set as the lower confidence limit of the Mean Time Between Failure (MTBF) to insure that the preventive maintenance is performed prior to equipment failure. However, statistically many equipments would have survived for a longer period but are overhauled prematurely because statistical methods apply to populations of similar equipments which can not examine each equipment individually and only calls for scheduled maintenance when this particular equipment needs it.
The RAMI is an electronic device that provides an early warning of impending equipment failure for each individual equipment which it monitors. The principle advantage of RAMI is that it is relatively accurate and deals with analyzing individual equipment as opposed to statistical methods which can only analyze populations of similar equipment. For example, assumming a launch fleet powered by diesel engines; suppose we wanted to calculate the probability of an engine failure before 10,000 hours by statistical means, the result would be exactly that, i.e., a "probability" but this would not mean that a specific engine would fail prior to, or subsequent to a specific time. Therefore, with respect to a specific unit of an entire population, its status cannot be determined unless it is individually examined. This is exactly what is done by RAMI--it provides an individual examination of each unit. However, it is different from other electronic monitors which indicate when a unit has failed because RAMI provides an early warning of impending failure in quantitative terms. This early warning period is a variable depending on the amount of time an equipment has been in service and its level of degradation in performance.
The advantages of an early warning period is that it provides for a planning interval to secure maintenance personnel, tools and replacement parts thus keeping the down time and attendant penalty costs at a minimum. This eliminates the shortcomings of human judgement which has directly or indirectly resulted in "avoidable" failures and skyrocketing maintenance costs. However, with RAMI, by quantifying typical parameters such as noise, vibration, temperature, pressure, tensile stress, etc., timely corrective actions can be taken as a result of their being continuously monitored and problem areas projected.
The following are some of the benefits which would accrue by using RAMI:
(a) Schedule preventive maintenance only when required as indicated by RAMI. PA1 (b) Optimize and control spare parts inventory. PA1 (c) Reduce Maintenance Costs. PA1 (d) Purchase spares and/or equipment for minimum total cost (and not only for minimum acquisition cost). PA1 (e) Isolate the equipment or parts, which are problem areas.